Method and apparatus for discharging polymerization reactors

ABSTRACT

A method and an apparatus of discharging polymer from a continuously operated gas phase reactor, wherein at least one monomer is polymerized in a bed containing active catalyst formed by catalyst and polymer particles suspended in a fluid, the bed defining a fluidized bed level in said reactor. The invention comprises continuously withdrawing polymer powder from the reactor; and adjusting the discharge rate of the polymer powder so as to maintain a constant bed level during polymerization. By means of the invention the discharge of the polymer can be made truly continuous without any disturbance of the polymerization. The rate of withdrawn polymer can be flexibly adjusted depending on the progress of the polymerization and it can also easily be scaled up if the capacity of the reactor is increased.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of producing polymersin a continuously operated polymerization reactor. In particular, theinvention concerns a method of continuously withdrawing solid polymerpowder from a fluidized bed polymerization reactor with high throughput.The present invention also concerns an apparatus for continuouswithdrawal of polymer powder from a fluidized bed polymerizationreactor.

[0003] 2. Description of Related Art

[0004] A number of processes for preparing polymers in a fluidized bedreactor are known in the art. Such processes are described in e.g. EPPatent Specification No. 0 517 868, U.S. Pat. No. 4,543,399 and EPPatent Application No. 0 381 364. The processes are primarily designedfor the production of polyethylene but they can also be modified forpreparing other polyolefins, such as polypropylene. In the processdescribed in EP Patent Specification No. 0 517 868 the fluidized bedreactor acts as a second reactor in a two-reactor train, in the otherabove-mentioned processes it is a stand-alone reactor.

[0005] Conventional gas phase fluidized bed reactors comprise anelongated reactor body generally having a vertical central axis. Themonomers are polymerized in a fluidized bed above a fluidization gridlocated in the bottom end of the reactor body. A gaseous streamcontaining monomer, and optionally alpha-olefin comonomer(s), hydrogenand inert gas(es) is introduced to the bottom of the bed through thefluidization grid. The unreacted gas is collected from the top of thebed, cooled and recycled to the bottom of the reactor. The polymerproduct is withdrawn from the lower part of the reactor above thefluidization grid. Active catalyst is introduced into the bed, either asa fresh catalyst or as polymer particles from a prior polymerizationstage, wherein the active catalyst is dispersed.

[0006] The polymerization system of a gas phase reactor used forpolymerization of α-olefins comprises a fluidized bed consisting ofpolymer particles containing the active catalyst within, and a gaseousreaction medium. The bed can be maintained in the fluidized state bymechanically mixing or stirring the contents of the reactor andadditionally or alternatively by blowing the monomer(s), i.e. theolefin(s), and/or an inert reaction medium (e.g. nitrogen and/or aneasily volatile hydrocarbon) into it in a gaseous state. In the case ofa fluidized bed process, the velocity of the gas needs to be sufficientto support or fluidize the polymer particles. The monomer(s) and/or aninert reaction medium in liquid state can be introduced into thepolymerization system and the polymerization can be carried out whilegasifying said monomer(s) and/or reaction medium. The unreactedmonomer(s) and/or inert reaction medium can be partly or whollyliquefied and recycled in liquid state into the polymerization system,as disclosed in EP-A1 0 024 933.

[0007] As disclosed in a U.S. Pat. No. 4,803,251, sheet formation isoccasionally encountered in gas phase reactors. The sheets are particleagglomerates having a thickness of about 6 to 15 mm, length of about 30to 150 cm and width of 7.5 to 45 cm or more. The sheets consist of fusedpolymer. According to U.S. Pat. No. 4,803,251, the formation of sheetsis associated with static electricity. The possible formation of hotspots, which eventually may lead to chunk formation as disclosed in EP 0089 691, was also discussed.

[0008] Conventionally, the withdrawal of the polymer product from a gasphase reactor is carried out batch-wise. A typical batch dischargeprocess consists of the following steps: When the surface level withinthe gas phase reactor rises as a result of polymerization, polymerpowder is discharged through an on/off valve to an outlet tank. A partof the fluidizing gas entering the tank with powder is recycled by acompressor back to the fluidized bed. The degassed powder is recoveredand optionally subjected to further processing.

[0009] By means of the batch-wise discharge system described above it ispossible to remove not only the polymer powder but also lumps formed inthe reactor, because the outlet nozzles of the system have a largediameter, generally much larger than required by the withdrawal of thepowder.

[0010] There are, however, some serious disadvantages associated withconventional technology. The batchwise discharge is a rather complicatedsystem. It comprises several phases and a number of on/off valves, whichtypically operate tens of times an hour. There has to be at least twodischarge systems, in large plants even more. This feature makes thebatch-wise discharge system rather expensive as an investment and alsothe maintenance is cost-consuming. The complexity of the system alsomakes it vulnerable; if the discharge system becomes clogged or there isa serious failure in the system, the whole plant has to be shut down.

[0011] Further, the batch-wise discharge is inflexible because it isdifficult to increase its capacity when the capacity of the whole plantis increased. Instead, a major rise in the production capacity requiresthe addition of a new expensive discharge system.

[0012] As regards the operation of the system, it can be noted thatthere is a distinct fluctuation in the bed level when a batch of thepolymer product is withdrawn from the reactor. This fluctuationinfluences the monomer concentration and, thus, also other parametres,such as the concentration of hydrogen and comonomers, which all takentogether have a strong impact on the the quality of the polymer product.

[0013] The discontinuous operation of the outlet system also leads to apulsating operation of the recycle gas compressor. This, again, leads toincreased wear.

[0014] Some alternative systems for withdrawal of polymer powder from afluidized bed reactor are also described in the art. Such processes aredescribed in EP Published Patent Applications Nos. 0 006 288 and 0 245043 and U.S. Pat. No. 4,495,337.

[0015] The last-mentioned document discloses a process for bottomdrainage of a fluid-bed reactor, in which the reactor is provided notonly with a vertical bottom pipe but also with a lateral evacuation pipelocated above the distribution plate of the reactor. The side pipe makesit possible to evacuate a part of the polymer formed in the reactor.According to the reference, to empty the reactor, first all the fractionof polymer powder located above the level of the outlet pipe isdischarged by means of the lateral pipe, then total drainage of thereactor is carried out via the vertical bottom pipe.

[0016] EP A1 0 006 288 and A1 0 245 043 disclose discharge systems whichare discontinuously or continuously operated. In EP A1 0 006 288 thedischarge is effected via a dipleg, which passes from the top of thereactor to a point near the bottom. The dip leg is connected to anon/off valve, which is opened intermittently. When the valve is open, amixture of polymer and monomer is passed on to a cyclone in which mostof the monomer vapour is separated from the polymer. The monomer can berecycled to the reactor via compressor.

[0017] EP A1 0 245 043 teaches an apparatus for degassing and pelletingpolyolefins, in which there is an outlet nozzle which is equipped with alock chamber placed between two valves activated alternately. Themixture of gas and solid particles, withdrawn from the reactor, istreated in a primary degasser for separating the solid phase from a partof the gas phase accompanying the particles. The gases containingmonomers are recycled into the reactor by a pipe.

[0018] Although the prior art suggests that the above three alternativedischarge systems can be not only periodically operated but alsocontinuously, none of them is in fact adapted for such operation. Itshould be noted that the control valves described in EP A1 0 006 288 andA1 0.245 043 are actually intermittently operated which means that thereis no continuous stream of polymer powder through the outlet nozzles.Thus, these systems are merely improved batch discharge systems, whichhave the same main disadvantages as described above. The dischargesystem of U.S. Pat. No. 4,495,337 is, on the other hand, suitable fordraining of the whole content of the reactor rather than for allowingfor continuous withdrawal of polymer powder.

[0019] JP-A-58/113208 discloses a process for continuously polymerizingolefins in vapour phase. The level of the fluidized bed is determined bymeasuring the pressure difference between two specified locations in thebed. The document further indicates that the polymer can be withdrawneither continuously or intermittently. How the suggested continuouswithdrawal is made in practice is not disclosed.

[0020] EP-A-0 870 539 discloses an apparatus for polymerizing olefins ingas phase. FIG. 2 and column 8, lines 21-55 of the document describe anembodiment, wherein the polymer is withdrawn continuously from thereactor. The document does not refer to the presence of any chunks orsheets among the polymer, and does not suggest how these should behandled when withdrawing polymer from the reactor. It merely discusseshow the gas discharge pipe should be installed to prevent it fromblocking.

SUMMARY OF THE INVENTION

[0021] It is an object of the present invention to eliminate theproblems related to the prior art of polymer powder discharge systemsand to provide a simple discharge method with high throughput, which iscapable of stabilizing fluidized bed polymerization reactor control anddownstream equipment operation.

[0022] Another object of the present invention is to provide a dischargesystem with low investment cost and substantially reduced maintenancecost.

[0023] These and other objectives, together with the advantages thereofover known processes, which shall become apparent from the followingspecification, are accomplished by the invention as hereinafterdescribed and claimed.

[0024] The present invention is based on the concept of polymerizing atleast one monomer in a gas phase reactor by continuously feeding amonomer into a bed formed by catalyst polymer particles suspended in afluid and defining a bed level-in the reactor. According to theinvention free-flowing polymer powder is continously with-drawn througha discharge pipe, while simultaneously monitoring the surface level ofthe fluidized bed within a gas phase reactor, and controlling the flowof material through the pipe depending on the surface level so as tomaintain an essentially constant bed level. To achieve this aim, the gasphase reactor is provided with an outlet nozzle which is equipped with acontinuously operated control valve for the powder discharge. Thecontrol signal for the control valve comes from a fluidized bed levelcontroller. The bed level is typically measured with a pressuredifference or radio-active device.

[0025] Further, the present invention comprises withdrawing andseparately (continuously or discontinuously) recovering particleagglomerates from the reactor.

[0026] The discharge system comprises an outlet nozzle, a control valveand a bed level indicator. Preferably the system further comprises acollecting vessel placed in communication with the outlet pipe forseparating gas from solid material. The particle agglomerates are eitherwithdrawn directly from the reaction via a separate outlet or theagglomerates are separated from the continuous flow of polymer powder.

[0027] More specifically, the method according to the present inventionis characterized by what is stated in the characterizing part of claim1.

[0028] The apparatus according to the present invention is characterizedby what is stated in the characterizing part of claim 26.

[0029] The present invention provides considerable advantages. Thus, thedischarge of the polymer can be made truly continuous without anydisturbance of the polymerization. The rate of withdrawn polymer can beflexibly adjusted depending on the progress of the polymerization and itcan also easily be scaled up if the capacity of the reactor isincreased. Further, it has been observed that a mixture of gas andpolymer can pass through the continuously operating control valve. Sucha mixture can be used for pneumatically conveying polymer powder. Also,it has been observed that less reaction gas is discharged than withbatchwise systems.

[0030] Particularly well the continuous outlet system is suited to aprocess where the outlet from the gas phase reactor is either directlyor indirectly fed into another gas phase reactor. This avoids thecontrol upsets in the next gas phase reactor due to the sequentialpowder feed.

[0031] Generally, it has not been deemed possible to achieve continuousdischarge because in particular in a PE reactor there are formedconsiderable amounts of lumps and clogs and the size of a continuouslyoperating outlet nozzle is so small that such lumps will eventuallycause clogging thereof. By the present invention it is possible to avoidsaid problem.

[0032] Next, the invention will be examined more closely with referenceto the attached drawings and a detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 shows in a schematical fashion a first embodiment of theinvention comprising a fluidized bed reactor with an outlet nozzle, apipe, a continuously operated control valve provided with means forflushing of the valve, and a processing section for separating gascontaining unreacted monomers from the polymer powder; and

[0034]FIG. 2 depicts in a schematical fashion a second embodiment of thepresent invention comprising a fluidized bed reactor with an outletnozzle, a collecting vessel, a control valve operated under the controlof a level indicator, and a processing section for separating gascontaining unreacted monomers from the polymer powder.

DETAILED DESCRIPTION OF THE INVENTION

[0035] According to the invention, the present method is used fordischarging polymer from a continuously operated gas phase reactor,wherein at least one monomer is polymerized in a bed containing catalystand polymer particles suspended in a fluid, said bed defining afluidized bed level in said reactor. The catalyst can be fed into thegas phase reactor as a stream comprising polymer and active catalysttogether with reaction medium. Such a stream can be obtained from aslurry reactor, such as a loop reactor.

[0036] The method comprises continuously withdrawing polymer powder fromthe reactor and adjusting the discharge rate of the polymer powder so asto maintain a constant bed level during polymerization. As mentionedabove, the discharge rate of the polymer powder is adjusted by using acontinuously operated control valve. Preferred examples of suchcontinuously operated valves are ball valves, V-ball valves and hosevalves.

[0037] To ensure stable operation of the process, the polymer powdershould be free-flowing. The flow properties of the polymer produced inthe process depend on the average particle size, particle sizedistribution and especially shape of the polymer particles. Theseproperties further depend on the corresponding properties of thecatalyst used in the process. To achieve good flow properties, thepolymer particles should have an average particle size between 150 and4,000 microns, preferably between 200 and 2,000 microns. The particlesize distribution should be such that no more than 30%, preferably nomore than 15% by weight of the particles should have a diameter smallerthan 100 microns. Moreover, the polymer particles should have a smoothsurface and a regular appearance. Preferably the particles should have aspherical or near spherical shape.

[0038] Together with or separately from the free-flowing polymer powder,polymer agglomerates are also withdrawn from the reactor and recovered.They may be discarded or milled and mixed with the ready polymer.Polymer “particle agglomerates” are particles having a minimum thickness(in any dimension) of at least about 6 mm, in particular about 6 to 15mm. They consist of polymer particles which are at least partially fusedtogether. The agglomerates comprise sheets, chunks and lumps formedduring polymerization, in particular in the gas phase reactor. Asmentioned above, polymer agglomerates will be formed in particularduring polymerization of ethylene.

[0039] The apparatus comprises an outlet nozzle communicating with thefluidized bed of the gas phase reactor, a collecting vessel placed incommunication with the outlet nozzle for separating gas from solidmaterial; a continuously operating valve for adjusting the amount ofpolymer powder withdrawn from the reactor via the outlet nozzle; andmeans for controlling the operation of the valve for adjusting thedischarge rate of the polymer powder so as to maintain a constant bedlevel during polymerization. The continuously operating valve ispreferably connected to the collecting vessel. Polymer agglomerates maybe withdrawn from the reactor using a discharge line with adiscontinuously operated discharge valve. Alternatively, polymeragglomerates are separated from the polymer powder in a collectingvessel provided with a screen for isolating the lumps, sheets or clogs.

[0040] A first preferred embodiment of the invention is shown in FIG. 1.In the drawing, the following reference numerals are used:

[0041]1. gas phase reactor

[0042]2. outlet nozzle of gas phase reactor

[0043]3. control valve

[0044]4. level indicator/controller

[0045]5. discharge line

[0046]7. collecting vessel

[0047]8. flush gas

[0048]12. valve regulating the flow of polymer powder

[0049]18 product receiver or second reactor

[0050]20 gas surge drum

[0051]21 compressor

[0052] As shown in the drawing, polymer formed in the reactor 1 iswithdrawn from a point below the bed level, via an outlet nozzle 2 and acontrol valve 3 together with a sufficient amount of gas for conductingthe polymer to a degassing vessel 18 or to a subsequent reactor. Theoperation of the control valve 3 is controlled by the levelindicator/controlling means 4. The gas separated from the polymer powderin the degassing section is recycled after the separation of the polymerto the gas phase reactor, as indicated by the arrow, and/or to arecovery section.

[0053] The continuous withdrawal is preferably used as a singledischarge system in case of a reactor wherein the amount of lumps issmall. The formation of lumps can be reduced by adding variousantistatic agents or by avoiding feeding fresh catalyst to the gas phasereactor, as in a multistep process. A particularly preferred multi-stepprocess is represented by the system disclosed in EP PatentSpecification No. 0 517 868, which comprises in combination a loopreactor and a gas phase reactor.

[0054] Even in this embodiment, it is advantageous to place the outletnozzle 2 at a point where no or practically no lumps are present. Tothat aim the polymer powder can be continuously withdrawn from a pointabove a fluidization plate, since the lumps tend to accumulate on thefluidization plate. When a gas phase reactor is used, wherein thefluidized bed comprises a mechanically mixed bed, the polymer powder ispreferably withdrawn from said mixed zone.

[0055] In order to reduce the risk of clogging of the discharge system,the outlet nozzle can be provided with a grid (not shown) flush mountedat the reactor wall to prevent lumps from entering the pipe. The term“flush mounted” means that the grid is installed so that it is at thesame level as the surface of the inner wall of the reactor. It must notextend from the wall surface to the reactor, otherwise it would becovered with polymer. Also, it must not leave a pocket at the nozzlewhich would allow accumulation of polymer there. Further the dischargeline 5 and the control valve 3 can be discontinuously backflushed with aflushing gas flow through gas conduit 8 to prevent clogging. Similarly,the control valve 3 can be adjusted to provide for pulsating operationto prevent clogging of the valve by a control means 4. Preferably, thisis done by opening the valve fully for a short time and then turning itback to its normal position. This procedure is automatically repeatedwith specified intervals.

[0056] Since it is normally not possible to avoid lump-forming totally,though it can be reduced to a minimum, as discussed above, it ispreferred to combine the present continuous discharge system with aconventional batchwise system, as shown in FIG. 1. In the presentembodiment the batch-wise discharge system is represented by acollecting tank 7, which is connected to the gas phase reactor via a gasconduit. By batch-wise withdrawal it is possible to prevent accumulationof lumps on the distribution plate of the gas phase reactor 1. In aprocess according to the present invention, the batchwise operatedsystem can be designed for a substantially smaller capacity thanconventionally and it can be operated less frequently, for example onlyonce an hour or even only once a day. Generally one batchwise outletnozzle should be enough for withdrawal of lumps (polymer aggregates).The lumps can be detected by radioactive detection sensors.

[0057] Typically, if the reactor is provided with a discontinuouspolymer discharge system, the ratio between the polymer powdercontinuously discharged from the reactor to polymer powder discharge viathe batchwise operated system is about 1:1 to 10,000:1.

[0058] The degassing of the polymer powder takes place in at least onedegassing vessel, wherein the pressure of the gas flow is decreased toremove the gas from powder. The degassing section 18, 20 can comprise aproduct receiver 18 for separating gas from the polymer powder and a gassurge drum 20. The polymer powder is pneumatically conducted from thecollecting vessel 7 to the product receiver 18 or a second reactor.

[0059] The pressure of the gas from the gas collecting vessel can beincreased in a compressor 21 before the gas is recycled to a gas phasereactor, directly or through a recovery system/section.

[0060]FIG. 2 shows another preferred embodiment for continuouswithdrawal. The parts of the equipment are largely the same as in theprevious embodiment, viz.:

[0061]31. gas phase reactor

[0062]32. outlet nozzle of gas phase reactor

[0063]33. control valve for gas

[0064]34. level indicator/controller

[0065]35. collecting tank

[0066]36. recycle line

[0067]37. screen

[0068]38. outlet nozzle to remove lumps from collecting tank

[0069]39. level indicator/controller

[0070]40. control valve to regulate the flow of powder

[0071]41. On-off valve for removal of lumps

[0072]43, 45 collecting tanks of degassing section

[0073]46 compressor

[0074] In the embodiment shown in FIG. 2, the outlet nozzle 32 of thegas phase reactor 31 is combined with a separate collecting vessel 35,which can be used for separating lumps from the fluid comprising polymerpowder and gas. The bed level of the gas phase reactor 31 is controlledby a recycle valve (control valve) 33 fitted in the recycle line 36connected to the the gas space of the collecting vessel. Gas is recycledto the upper part of the reactor above the fluidized bed and the bedheight gives the necessary pressure difference for conducting thepolymer and the gas to the collecting vessel 35.

[0075] The collecting vessel 35 is preferably provided with a separatelevel controller 39 and a control valve 40 and polymer is pneumaticallyconducted under the influence of the pressure of the collecting vessel35 to a degassing vessel 43 or to a subsequent reactor. It is preferredto provide the collecting vessel with a screen 37 for collecting thelumps and with a separate discharge nozzle 38 for the lumps. The controlvalve 40 may be operated either intermittently or continuously. It ispreferred to operate the valve 40 continuously.

[0076] The operation of the degassing section 43-46 is similar to thatof the first embodiment explained above.

[0077] The polymer continuously withdrawn can be directly or-indirectlyfed into another gas phase reactor.

[0078] The present invention can be used for producing homo- andcopolymers from monomers selected from the group of C₂ to C₁₆ olefinsand mixtures thereof. Preferably the monomer is ethylene, propylene,1-butene, 4-methyl-1-pentene, 1-hexene, a diene, or cyclic olefin, or amixtures thereof.

[0079] The following non-limiting Examples illustrate the invention.

COMPARATIVE EXAMPLE

[0080] A production scale gas phase reactor was operated at 85° C.temperature and 20 bar pressure with a bed level of 15 m as follows:

[0081] Polymer containing active polymerization catalyst was fed intothe gas phase reactor from a loop reactor at a rate of 6 metric tons perhour. The feed rates of ethylene, hydrogen and 1-butene were adjusted sothat desired concentrations in the gas phase reactor were reached. As aconsequence, the polymer production rate in the gas phase reactor was 8tons per hour and the polymer taken out of the reactor had an MFR₂, =11and density=947 kg/m³.

[0082] The polymer was collected from the reactor using a conventional,batchwise product outtake system, where the polymer was collected into a3.7 m³ receiving tank. Two identical systems were used. The diameter ofthe outtake nozzle was 8 inches. The period between two successiveouttakes was two minutes, i.e. four minutes for each individual system.During a normal operation the bed level was found to oscillate with a 20cm amplitude. The reactor could be operated several days without ashut-down.

[0083] An experiment to operate the product outtake with its maximumcapacity was made, and then the capacity was found to be 20 tons perhour.

EXAMPLE

[0084] A product outtake system according to the present invention wasinstalled in the gas phase reactor of the Comparative example. Thediameter of the nozzle was 2 inches (50 mm). Every minute the controlvalve was fully opened for a few seconds to prevent clogging of thevalve. The conventional discontinuous outtake was set to operate once anhour to remove any lumps from the reactor. No lumps were actuallycollected from the reactor during the test period and it was thusconcluded that the operation frequency of the discontinuous outlet couldhave been even lower.

[0085] The conditions of the reactor were similar to the Comparativeexample. During the normal operation the bed level was found tooscillate with a 5 cm amplitude. The reactor was operated several daysin a stable fashion until the test was interrupted.

[0086] In an experiment according to the Comparative example the maximumcapacity of the outtake was found to be 45 tons per hour.

1. A method of producing a polymer in a continuously operated gas phasereactor, comprising polymerizing at least one monomer in a bedcontaining active catalyst formed by catalyst and polymer particlessuspended in a fluid, said bed defining a fluidized bed level in saidreactor, continuously withdrawing polymer powder from the reactor;adjusting the discharge rate of the polymer powder so as to maintain aconstant bed level during polymerization; and withdrawing and separatelyrecovering particle agglomerates from the reactor.
 2. The methodaccording to claim 1, wherein the discharge rate of the polymer powderis adjusted by using a continuously operated control valve.
 3. Themethod according to claim 2, wherein the continuously operated valve isa ball valve, a V-ball valve or a hose valve.
 4. The method according toclaim 2 or 3, wherein the polymer powder is withdrawn via an outletnozzle connected to the control valve, and said nozzle is provided witha grid flush mounted at the reactor wall to prevent lumps from enteringthe pipe.
 5. The method according to any of claims 2 to 3, wherein theoperation of the control valve is adjusted by using a control signalobtained from a bed level controller.
 6. The method according to any ofclaims 2 to 5, wherein the control valve is adjusted to provide forpulsating operation to prevent clogging of the valve.
 7. The methodaccording to any of the preceding claims, wherein polymer powder iscontinuously withdrawn from a point above a fluidization plate.
 8. Themethod according to any of the preceding claims, wherein polymer powderis continuously withdrawn from a point below the bed level.
 9. Themethod according to any of the preceding claims, wherein the dischargeline and the control valve are discontinuously backflushed with aflushing gas flow.
 10. The method according to any of the precedingclaims, comprising using a gas phase reactor having a mechanically mixedzone of the fluidized bed, and continuously withdrawing polymer powderfrom said mixed zone. 11 The method according to any of the precedingclaims, wherein polymer powder is also separately withdrawn from thereactor using a discontinuous discharge device.
 12. The method accordingto any of the preceding claims, wherein the polymer powder is withdrawntogether with gas from the reactor, the gas is separated from thepolymer powder, and the separated gas is recycled into the reactor. 13.The method according to any of the preceding claims, wherein polymeragglomerates are withdrawn from the reactor using a discharge line witha discontinuously operated discharge valve.
 14. A method of dischargingpolymer from a continuously operated gas phase reactor, wherein at leastone monomer is polymerized in a bed containing active catalyst formed bycatalyst and polymer particles suspended in a fluid, said bed defining afluidized bed level in said reactor, comprising continuously withdrawingpolymer powder from the reactor; feeding the withdrawn polymer powderinto a collecting vessel, wherein lumps are separated fromfinely-divided polymer powder and at least a part of the gas isseparated from the solid material; recovering the lumps, and adjustingthe discharge rate of the polymer powder so as to maintain a constantbed level during polymerization.
 15. The method according to claim 14,wherein the separated gas is recycled into the reactor, said collectingvessel being provided with a return valve for adjusting the gas flowrecycled to the reactor.
 16. The method according to claim 15, whereinthe return valve is controlled by the fluidized bed level of thereactor.
 17. The method according to claim 16, wherein the polymer levelin the vessel is controlled by using a continuously operating controlvalve.
 18. The method according to any of claims 14 to 17, wherein thecollecting vessel is provided with a screen for separating lumps. 19.The method according to any of the preceding claims, wherein thecatalyst is fed into the gas phase reactor as a stream comprisingpolymer and active catalyst together with reaction medium.
 20. Themethod according to claim 19, wherein the catalyst is fed into the gasphase reactor from a slurry reactor.
 21. The method according to claim20, wherein the slurry reactor is a loop reactor.
 22. The methodaccording to any of the preceding claims, wherein the monomers areselected from the group of C₂ to C₁₆ olefins and mixtures thereof. 23.The method according to any of the preceding claims, wherein the monomeris selected from the group of ethylene, propylene, 1-butene,4-methyl-1-pentene, 1-hexene, dienes, and cyclic olefins, and mixturesthereof.
 24. The method according to any of the preceding claims,wherein the polymer that is continuously withdrawn is either directly orindirectly fed into another gas phase reactor.
 25. The method accordingto any of claims 14 to 24, wherein the collecting vessel is connected toa gas separator, said polymer powder being pneumatically conducted fromthe collecting vessel to the gas separator.
 26. An apparatus fordischarging polymer from a continuously operated gas phase reactor,wherein at least one monomer is polymerized in a bed containing activecatalyst formed by catalyst and polymer particles suspended in a fluid,said bed defining a fluidized bed level in said reactor, said appartuscomprising an outlet nozzle communicating with the fluidized bed of thegas phase reactor, a collecting vessel placed in communication with theoutlet nozzle for separating gas from solid material, with a screen forcollecting the lumps and a separate discharge nozzle for the lumps; acontinuously operating valve for adjusting the amount of polymer powderwithdrawn from the reactor via the outlet nozzle; and means forcontrolling the operation of the valve for adjusting the discharge rateof the polymer powder so as to maintain a constant bed level duringpolymerization.
 27. The apparatus according to claim 26, wherein thecontinuously operating valve is connected to the collecting vessel. 28.The apparatus according to claim 26 or 27, wherein the collecting vesselcomprises a gas space which is connected to the gas phase reactor via agas conduit.